This invention relates to high purity hydrogen ion buffers. More particularly, this invention relates to amino-organosulfonic acid zwitterionic compositions having low metal content which are particularly desirable for use in the electronics industry.
A group of hydrogen ion buffers has been discussed by Good et al., in Biochemistry, Volume 5, No. 2, pages 467-477, 1966. Some of these buffers may be generally classified as substituted amino-organosulfonic acid zwitterions. These materials have a variety of uses including pH control in printing inks, bioprocessing and electronic component manufacture. However, in the electronics industry, the materials used must have at most, very low levels of impurities, such as metals ranging from alkali to silicon whose presence may increase the propensity of component failure. There continues to be a need for high purity amino-organosulfonic acid zwitterionic buffers with very low metal concentrations for use in the electronics industry.
The present invention comprises an amino-organosulfonic acid zwitterionic composition comprised of an amino-organosulfonic acid zwitterion having the general structure: 
wherein R1 and R2 are each independently a hydrogen atom, an aliphatic, cycloaliphatic, substituted aliphatic, substituted cycloaliphatic, aryl, substituted aryl, heterocyclic group or substituted heterocyclic group, or R1 and R2 are joined to form a cycloaliphatic, substituted cycloaliphatic, aryl, substituted aryl, heterocyclic group or substituted heterocyclic group, wherein R1 and R2 preferably range from C1 to C20 and more preferably from C1 to C10, or together form a substituted heterocyclic group. more preferred is a substituted piperazine such as 2-hydroxyethylpiperazine; and R is an aliphatic, cycloaliphatic, hydroxyaliphatic, or an aryl group ranging from C1 to C20 and preferably aliphatic ranging from C2 to C4; and wherein the concentration of any single metal in the composition is no greater than about 500 ppb, and preferably is less than about 200 ppb, more preferably is less than about 150 ppb, even more preferably is less than about 100 ppb, most preferably is less than about 50 ppb, and ideally is less than about 20 ppb. These metals are selected from the group consisting of aluminum, antimony, barium, boron, cadmium, calcium, chromium, cobalt, copper, gallium, germanium, gold, iron, lead, magnesium, manganese, nickel, potassium, silicon, silver, sodium, strontium, tantalum, tin, and titanium and mixtures thereof. Each of the metals may be present individually, or any combination thereof. There is no total metal concentration requirement, but each metal present must be present at no greater concentration than about 500 ppb.
Examples of amino-organosulfonic acid zwitterions include, but are not limited to 4-(2-Hydroxyethyl)piperazine-1-ethanesulfonic acid, 4-Morpholinepro-panesulfonic acid, b-hydroxy-4-morpholinepropanesulfonic acid, 2-(N-Morpholino)-ethanesulfonic acid, 1,4-piperazinebis(ethanesulfonic acid), N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic acid, 2-[(2-Hydroxy-1,1-bis[hydroxymethyl]ethyl)-aminoethane-sulfonic acid, and 2-(Cyclohexylamino)ethanesulfonic acid.
The present invention also relates to a method of producing a composition which comprises an amino-organosulfonic acid zwitterion and wherein the concentration of any single metal in the composition has a concentration no greater than about 500 ppb, preferably less than 200 ppb, more preferably less than about 150 ppb and even more preferably, less than about 100 ppb, most preferably less than about 50 ppb and ideally less than about 20 ppb. The method for achieving low metal concentrations consists of the following steps: (i) dissolving the amino alkyl sulfonate in an aqueous solution; (ii) flowing the aqueous solution through an iminodiacetic ion-exchange resin; (iii) flowing the aqueous solution through an electrodialysis apparatus comprised of a two-compartment cell configuration having anion and cation exchange membranes; (iv) flowing the aqueous solution through a basic ion-exchange resin wherein the basic ion-exchange resin has a pH of at least 9; (v) flowing the aqueous solution through a mixed bed ion-exchange resin; and (vi) filtering the aqueous solution through a filter having a pore size of about 0.5 microns to about 0.2 microns and is preferably about 1 micron. The order of these steps (ii)-(v) is not critical and they may be carried out in any sequence. The amount of de-ionized water the sample may be dissolved in varies from a concentration of from about 9:1 to about 1:9, by weight (sulfonic acid: water).